1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus free from limitations on thin-down reading in a four-field interline transfer system, and a method of reading signals out of the same. More particularly, the present invention relates to a four-field interline transfer type solid-state image pickup apparatus capable of picking up high definition images and advantageously applicable to, e.g., an electronic still camera or a video camera.
2. Description of the Background Art
A high definition TV (television) system dealing with images about two times higher in vertical resolution than images available with the current TV broadcasting system has recently been proposed. Also, electronic still cameras improved in vertical resolution are presently under development. An electronic still camera with improved vertical resolution uses interline transfer type photosensitive devices or cells doubling the number of pixels in the vertical direction and allowing all of its pixels to be read out by four consecutive times of field scanning. Japanese patent No. 2660594, for example, discloses an electronic still camera using photosensitive cells forming a great number of pixels and freeing reproduced images from flicker and other defects.
The prerequisite with an electronic still camera is, of course, that an image be recorded with high definition. On the other hand, high vertical resolution is not necessary when an image is displayed on the conventional electronic viewfinder, LCD (Liquid Crystal Display) or similar display. For this kind of display, it is necessary to thin down lines in the vertical direction for a power saving purpose, among others.
Further, there is an increasing demand for an electronic still camera with a greater number of pixels, smaller pixel size, and lower drive voltage for shooting. However, an electronic still camera meeting such a demand would further lower the amount of signal charge capable of being stored in the individual photosensitive cell. It is well known that when extraordinary intense light or similar unusual light is incident to any one of such photosensitive cells, a signal charge exceeding the charge storing capacity of the photosensitive cell overflows the device and turns out a false signal referred to as blooming. To solve the blooming problem, each photosensitive cell may be provided with an overflow drain for absorbing the false signal in a substrate. While this kind of structure improves the above situation to a significant degree, it sacrifices the photosensitive area and therefore sensitivity of the individual photosensitive cell. Japanese patent laid-open publication Nos. 176236/1993 and 29814/1995 each propose a method of reducing false signals by signal processing without lowering the sensitivity of the photosensitive cells.
The four-field interline transfer type photosensitive cells mentioned earlier each are caused to output a signal charge to a vertical transfer path via a transfer gate by field-by-field scanning (field shift). The vertical transfer path sequentially transfers the signal charge vertically in accordance with a vertical drive signal. The vertical transfer path has a capacity designed to accommodate a signal charge output from at least one photosensitive cell. To read out the entire pixels with the above type of photosensitive cells, the field scanning is repeated four times. Consequently, a time for reading out the signal charge in an image pickup period of time increases with an increase in the number of pixels and lowers an image refreshing rate. For example, in a monitor mode for displaying a scene on the electronic viewfinder of an electronic still camera before a shot, it may occur that an image appearing on the viewfinder is not coincident with the actual scene. This renders the camera inconvenient to use. Moreover, when color filters for separating the colors of incident light are arranged in a Bayer pattern, not all of three primary colors R (red) G (green) and B (blue) can be output line by line even if the lines are read out during a field time.
Assume that signals are read out of the above photosensitive cells while being thinned down in order to display a desirable image in, e.g., the monitor mode. Then, two fields of signal charges are transferred to the vertical transfer paths at a time. However, the transfer capacity available with each vertical transfer path is substantially one half of the amount of signal charge to be read out and is therefore short at the time of thin-down reading. It follows that thin-down reading is limited in the four-field interline transfer system.
It is therefore an object of the present invention to provide a solid-state image pickup apparatus free from limitations on thin-down reading in, e.g., the four-field interline transfer system.
A solid-state image pickup apparatus of the present invention includes an image pickup section. The image pickup section includes, optics for focusing incident light representative of a scene, a color separating section for separating the colors of light incident via the optics, a plurality of photosensitive cells each for transforming light incident via the color separating section to a corresponding signal charge, a plurality of vertical transfer paths each transfers the signal charges fed from adjoining ones of the photosensitive cells in the direction of columns, signal reading circuitry for selecting the reading of the signal charges from the photosensitive cells to the vertical transfer paths, and a horizontal transfer paths for transferring the signal charges fed from the vertical transfer paths in the direction of rows. A mode setting section selects and sets one of a plurality of modes each matching with a particular usage of the signal charges output from the photosensitive cells. A drive signal generating section generates drive signals to be fed to the photosensitive cells, vertical transfer paths, signal reading circuitry and horizontal transfer path for outputting signals representative of a single picture in a plurality of fields in accordance with the mode set by the mode setting section. A controller controls the drive signal generating section in accordance with the output of the mode setting section. An amplifying circuit amplifies the outputs of the image pickup section. The drive signal generating section includes a plurality of timing signal generating circuits each for generating signals for transferring the signal charges in the direction of columns by a particular number of phases corresponding to the mode set, a first potential feeding circuit for controlling a substrate potential determining the signal charge capacity of each photosensitive cell to a first preselected potential, a second potential feeding circuit for causing the substrate potential to rise to a second preselected potential higher than the first preselected potential, and an output selecting circuit for selecting, under the control of the controller, the outputs of one of the timing signal generating circuits, first potential feeding circuit and second potential feeding circuit in accordance with the mode set.
Also, a signal reading method using an interline transfer system for sequentially reading out, in accordance with drive signals, signal charges produced by photoelectric conversion of incident light representative of a scene by repeating a field shift, a transfer in the direction of columns and a transfer in the direction of rows to thereby transform an image represented by the signal charges to a color image higher in definition than an image available with the standard broadcasting system of the present invention begins with a step of selectively setting a first shoot mode for generating the color image having high definition or a second shoot mode for generating the image available with the standard broadcasting system. Control signals matching with the first shoot mode or the second shoot mode set are generated. Drive signals for the field shift, transfer in the direction of columns and transfer in the direction of rows are generated in accordance with the control signals. A plurality of photosensitive cells for generating the signal charges are prepared. Drive signals are generated for reducing, in the second shoot mode, the saturation capacity of each photosensitive cell to one-third or one-half of a saturation capacity particular to the first shoot mode, and the drive signals matching with the mode set are selected. The incident light is converted to the signal charges. When a plurality of field shifts are effected at the same time, there is selected the drive signals capable of maintaining a preselected distance between transfer elements to be read matching with the mode selected. The signal charges transferred to the above transfer elements are transferred in the direction of columns. The transfer in the direction of columns is repeated to transfer the signal charges line by line, and then the signal charges are transferred in the direction of rows. Finally, signals output by the transfer in the horizontal direction are amplified.